Interpret the results.
There may be other gases present in the confined space that interfere with the reading for the gas you are trying to measure. Such gases are referred to as “interfering gases”: They can lead to misinterpretation of the monitoring results.
Know the limitations of your specific equipment. Consult the manufacturer’s instructions for proper use.
Temperature, humidity, and interfering gases can all affect the performance of gas monitors.

If the atmosphere meets acceptable exposure limits the
confined space may be entered. If the atmosphere does not meet acceptable limits, you need to implement controls before anyone can enter.

Acceptable Atmospheric Levels

Oxygen:      Between 19.5% and 23%

Explosive or Flammable Gas or Vapour

<25% of lower explosive limit: inspection work can be performed

<10% of its lower explosive limit: cold work can be performed. (Cold work is work which does not involve welding or cutting, the use of tools which could produce a spark or other sources of ignition.)

<5% of its lower explosive limit: hot work can be performed

Exposure to Atmospheric Contaminants (Toxic)

Exposure to toxic contaminants must not exceed permissible exposure limits (PEL). Exposure limits can be found in the regulations REG 833 Control of Exposure to Biological and Chemical Agents and the Designated Substances Regulations.

Never trust your senses to determine whether the atmosphere in a
confined space is safe.
 You cannot see or smell many toxic gases and vapours.
 You cannot determine by your senses the level of oxygen present.
 Know which gases or vapours may be present in the confined space and test for each of them.